The Taliban has issued a warning that it will increase attacks on foreign military forces in Afghanistan. NBC's Ron Mott reports.

By Ron Mott, Correspondent, NBC News

KABUL — It didn’t take long. Within hours of announcing the start of its annual spring offensive, the Taliban in Afghanistan claimed responsibility for an early-morning attack in the eastern part of the country that killed at least three police officers.

"In addition to suicide bombings, insurgents warned of coordinating ‘insider’ attacks against ‘foreign transgressors’," the Taliban said in a statement on Sunday.

The Taliban, known for employing bombastic language in describing its achievements--claims frequently invalidated or unproven--hailed the 2013 spring offensive as "monumental.”

This year, the Taliban's annual declaration of increased violence--coinciding with the break from harsh winter weather--is widely considered an especially crucial test for President Hamid Karzai's government as it prepares to assume control of the nation's security from coalition forces, which are slated to withdraw combat troops in 2014.

Abdul Mueed / EPA

Security officials check a car in Jalalabad, Afghanistan, on Sunday. Security was intensified following the announcement by Taliban militants that they were launching their spring offensive.

That challenge has been particularly lethal so far in April as Afghan security forces increasingly take the lead in the 11-year war. The month began with an ambush of a courthouse in western Afghanistan by nine suicide attackers disguised as soldiers that left at least 44 dead, injuring more than 100.

According to The Associated Press, 478 people—217 of these insurgents—have been killed in violence around the country so far in April. A total of 447 people, including 268 insurgents, were killed during the same months in 2012, according to the AP.

‘NATO has done nothing’In the capital Sunday, storefronts were largely shuttered for Victory Day, commemorating the 1992 defeat of Communist rule, but tailor Zulmai Mohamadi was open for business, lamenting the prolonged conflict and its impact on daily life.

“NATO has done nothing for us,” the 38-year-old father of seven said. “In the past 11 years, what can I say? They have done nothing. They did whatever they did for their own interests and not for us.

“Our poor nation is in the same condition of poverty and all those problems. I think the future will remain the same in future years: poverty with a lot of problems.”

“We have our own security forces, the Afghan police and army,” said 28-year-old Sofia Farkhunda. “They are working for the community. They have controlled the situation very well. That’s why I’m hopeful that everything will go well because our police are better prepared and trained.”

The International Security Assistance Force (ISAF), the NATO-led group involving some 66,000 American troops, echoed that confidence.

"The Afghan people should not fear Taliban threats," ISAF said in a statement. "The Afghan National Security Forces have wisely used the winter months to prepare for taking over the security lead throughout Afghanistan by mid-2013. They are ready."

New Taliban offensiveThe Taliban dubbed its operation "Khalid bin Waleed," honoring the Islamic general known as the "Drawn Sword of God" and a companion of the prophet Muhammad.

"We once again call on all the officials and workers of the stooge Karzai regime to break away from this decaying administration in order to conform to Islamic commands, national interests and protection of yourselves, and to choose a life of prosperity living alongside your own people in an atmosphere of peace and security," the Taliban said in its statement.

NBC News' Akbar Shinwari and Kiko Itasaka, and The Associated Press contributed to this report.

Rahmatullah Alizad / AFP - Getty Images

Men carry the coffins of police officers who were killed in a roadside bomb ambush in Afghanistan's Ghazni province on Sunday.